Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session D59: Magnetic Topological Insulators 
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Sponsoring Units: DMP Room: Mile High Ballroom 3C 
Monday, March 2, 2020 2:30PM  2:42PM 
D59.00001: Localized and collective magnetic excitations in the magnetic topological insulator Sn_{1x}Mn_{x}Te Robert McQueeney, David Vaknin, Santanu Pakhira, Daniel M. Pajerowski, David C Johnston, Deborah Schlagel The nature of the magnetism dilute magnetic topological insulators (MTIs), controlled by atomic disorder and/or clustering and the competition between short and longrange magnetic interactions, is not well understood. MTIs that develop ferromagnetic (FM) longrange order can host dissipationless electronic transport via the quantum anomalous Hall effect. We studied the magnetic excitations in a prototypical MTI, Sn_{0.95}Mn_{0.05}Te, using inelastic neutron scattering. Neutron diffraction and magnetization data indicate that our Sn_{0.95}Mn_{0.05}Te sample has no FM longrange order. However, we observe slow, collective FM fluctuations (<70 μeV), indicating proximity to FM order. We also find a series of sharp peaks originating from excitations of antiferromagnetically coupled MnMn dimers with J_{AF }=230 μeV. These dimers sit at nextnearest neighbor positions on the FCC sublattice, suggesting that Mn substitution is not completely random. The simultaneous presence of collective and localized components indicate that some Mn ions contribute to FM order and some form stronglybound dimer singlets. 
Monday, March 2, 2020 2:42PM  2:54PM 
D59.00002: Topological insulator interfaced with ferromagnetic insulators: Bi_{2}Te_{3} thin films on magnetite and iron garnets ChiNan Wu, Vanda M Pereira, Simone G Altendorf, ShengChieh Liao, ChengEn Liu, Alexander Komarek, Mengxin Guo, HongJi Lin, ChienTe Chen, Minghwei Hong, Jueinai Kwo, Liu Tjeng We have grown Bi_{2}Te_{3} thin films on Y_{3}Fe_{5}O_{12}(111), Tm_{3}Fe_{5}O_{12}(111), Fe_{3}O_{4}(111), and Fe_{3}O_{4}(100) by molecular beam epitaxy with minimal chemical reaction at the interface. Electrical transport measurements were performed to study the magnetism induced by the proximity effect in the topological insulator in conjunction to the ferromagnetic insulators. We observed the anomalous Hall effect on these heterostructures. Magnetoresistance measurements at low temperature reveal a suppression of the weak antilocalization, indicating a possible topological surface state gap opening induced by the magnetic proximity effect. However, we did not observe any obvious xray magnetic circular dichroism (XMCD) on the Te M_{45} edges. The results suggest that the ferromagnetism induced by the magnetic proximity effect via Van der Waals bonding in Bi_{2}Te_{3} by is too weak to be detected by XMCD, but still can be observed by electrical transport measurements. This is consistent with published densityfunctional theory results on topological insulator/magnetic insulator heterostructure showing that only a small band gap of 9 meV is induced by the magnetic proximity effect. 
Monday, March 2, 2020 2:54PM  3:06PM 
D59.00003: Tailoring Hybrid Anomalous Hall Response in Engineered Magnetic Topological Insulator Heterostructures Peng Chen, Yong Zhang, Qi Yao, Thorsten Hesjedal, Shilei Zhang, Xufeng Kou Magnetic topological insulators (MTIs) have greatly broadened the research scope of topological quantum materials. Introducing MTIs into the field of spintronics defines a new trend of magneticbased logic and memory applications. Engineering the anomalous Hall effect (AHE) in emerging MTIs has great potentials for quantum information processing and spintronics applications. 
Monday, March 2, 2020 3:06PM  3:18PM 
D59.00004: Molecular Beam Epitaxy Growth and Magnetization Characterization of Fedoped Bi2Se3 Zheng Ren, He Zhao, Hong Li, Bryan Rachmilowitz, Ilija Zeljkovic Bi_{2}Se_{3} is a prototypical 3D topological insulator (TI), which hosts gapless surface states protected by timereversalsymmetry (TRS). When TRS is broken by doping with magnetic elements, this system can realize exotic electronic states, such as the quantum anomalous hall states (QAHS) (Rui Yu et al., Science 329 , 61 (2010)). We grow Fedoped Bi_{2}Se_{3} thin films on SrTiO_{3}(001) substrates by molecular beam epitaxy (MBE), and characterize them using a combination of lowtemperature scanning tunneling microscopy/spectroscopy (STM/S) and magnetization measurements. We find a large difference between the Fe concentration in the topmost layer determined from STM topographs and that inferred from magnetization measurements. Moreover we find an intriguing magnetic anisotropy of the thin films, different from that observed in their bulk counterparts. Our findings provide a fresh insight into the idea of doping Fe into Bi_{2}Se_{3} as a route towards achieving QAHS. 
Monday, March 2, 2020 3:18PM  3:30PM 
D59.00005: Optically manipulating ferromagnetism in Crdoped topological insulators (TIs) Adrian Llanos, ChienChang Chen, Marcus L Teague, Xiaoyu Che, Peng Zhang, Lei Pan, Kang L. Wang, NaiChang Yeh Optically manipulating ferromagnetic materials has been shown to be a promising route to optospintronic applications. Using a combination of opticallyenabled transport measurements and scanning tunneling spectroscopic (STS) measurements in the presence of circularly polarized (CP) light, we found an enhancement of magnetization in 10% Crdoped (Bi_{x}Sb_{1x})_{2}Te_{3} bilayer heterostructures which consisted of a pure layer (Bi_{x}Sb_{1x})_{2}Te_{3 }on top of a 10% Crdoped (Bi_{x}Sb_{1x})_{2}Te_{3 }layer. Measurements of the anomalous Hall resistance revealed an increase in R_{xy} and a decrease in longitudinal resistance R_{xx} in the bilayer magnetic TI system under CP light (wavelengths λ = 1600 ~ 1700 nm). In contrast, both R_{xx} and R_{xy} were suppressed under CP light for uniformly Crdoped (Bi_{x}Sb_{1x})_{2}Te_{3}. To understand the microscopic origin of these results, we performed spatially resolved STS studies on the surface state of magnetic TIs as a function of temperature and magnetic field. We further conducted opticallyassisted STS studies to spatially map out the CP lightinduced spectral changes to the surface state. The physical implications from correlating the spatially resolved STS under CP light with findings from macroscopic R_{xx} and R_{xy} will be discussed. 
Monday, March 2, 2020 3:30PM  3:42PM 
D59.00006: Magnetotransport properties of bulkinsulating topological insulators (Bi,Sb)_{2}Te_{3} on thulium iron garnets ChunChia Chen, Shang Rong Yang, YuTing Fanchiang, WeiJhih Zou, Mengxin Guo, ChaoKai Cheng, ShengWen Huang, KengYung Lin, KoHsuan Chen, Minghwei Hong, Jueinai Kwo Breaking timereversal symmetry in topological insulators (TIs) via magnetic proximity effect attracted intense studies. The spin dynamics of magnetic insulators (MIs) interfacing with TIs has been investigated by ferromagnetic resonance,^{1} and the magnetotransport of magnetized Bi_{2}Se_{3} by MIs has been reported,^{2} yet the crucial bulkinsulating TI, (Bi,Sb)_{2}Te_{3} (BST) requires further thorough study. In this work properties of BST thin films deposited by molecular beam epitaxy on aAl_{2}O_{3} and tensilestrained Tm_{3}Fe_{5}O_{12} (TmIG) of perpendicular magnetic anisotropy are reported. By adopting the lowtemperature growth method,^{3} we demonstrated a significantly improved film growth evidenced by streaky reflection highenergy electron diffraction patterns attained at the first quintuple layer of BST on TmIG. TIs were confirmed bulkinsulating by electric transport and angleresolved photoemission spectroscopy. We observed anomalous Hall effect up to 300K accompanied by suppressed weak antilocalization. Our work promotes the realization of quantum anomalous Hall effect at higher temperature and future TIbased devices. 
Monday, March 2, 2020 3:42PM  3:54PM 
D59.00007: Hightemperature ferromagnetic topological crystalline insulating state induced by proximity effect in a EuS/SnTe heterostructure Ryota Akiyama, Kazuki Watanabe, Yuta Tomohiro, Takeru Shimano, Ryo Ishikawa, Kazuhiro Akutsu, Kazuki Iida, Shinji Kuroda, Shuji Hasegawa Ferromagnetic topological insulators attract much attention because they enable us to realize the quantum anomalous Hall effect (QAHE), possibly useful for lowenergyconsumption devices by using the chiral edge state. However, the temperature for QAHE is still low mostly due to degradation of crystallinity by doping magnetic atoms. To prevent it, using ferromagnetic proximity effect is one of the solutions [1,2]. We made and measured a EuS/SnTe heterostructure. As a result, intriguingly, the perpendicular magnetization (PM) reaches minimum at ~100 K and then increases with increasing temperature and keeps up to room temperature. A similar behavior was reported in EuS/Bi_{2}Se_{3}[1], and the explanation is that after disappearance of bulk magnetization in EuS (T_{C}=17 K), the direction of the interface magnetization becomes perpendicular from oblique with increasing temperature. On the other hand, when we use a trivial insulator PbTe: EuS/PbTe, the PM monotonically decreases with increasing temperature. This suggests that the anomalous interface ferromagnetism is induced by the nontrivial nature in SnTe. [1] F. Katmis et al., nature 533, 513 (2016). [2] R. Akiyama et al., arXiv 1910.10540 (2019). 
Monday, March 2, 2020 3:54PM  4:06PM 
D59.00008: Ultrafast momentumresolved study of electronphonon coupling in an antiferromagnetic topological insulator Haricharan Padmanabhan, Vladimir A Stoica, Huaiyu Wang, Nathan Koocher, Mingqiang Gu, Xiaozhe Shen, MingFu Lin, Seng Huat Lee, Zhiqiang Mao, Aaron Lindenberg, Xijie Wang, James Rondinelli, Venkatraman Gopalan Elementary electronic and lattice excitations and their mutual interactions form the foundation of our understanding of condensed matter systems. In the context of topological insulators, the electronphonon coupling determines in addition, the robustness of dissipationless surface states at finite temperatures. In this work, we consider the first discovered intrinsic antiferromagnetic topological insulator, MnBi_{2}Te_{4}, a system that is predicted to exhibit the quantum anomalous Hall effect [1]. We study the momentumresolved electronphonon coupling in this material at its inherent femtosecond timescale using ultrafast electron diffraction and coherent phonon optical spectroscopy. We find that electrons are strongly coupled to inplane zoneboundary Eg optical phonons, resulting in a highly nonequilibrium phonon population for several hundreds of femtoseconds after excitation. The nonequilibrium phonon system subsequently relaxes by phononphonon coupling to zonecenter transverse acoustic phonons. We simulate how the strongly coupled Eg phonons modulate the exchange interaction and magnetism using DFT calculations. 
Monday, March 2, 2020 4:06PM  4:18PM 
D59.00009: Pressurizing an antiferromagnetic topological insulator candidate Priscila Rosa, Sean Thomas, Eric Bauer, Joe D Thompson, Filip Ronning Intrinsic magnetic topological insulators hold the potential of hosting quantum anomalous Hall states, chiral Majorana fermions, and topological magnetoelectric effects. We have identified Zintl antiferromagnet Eu_{5}In_{2}Sb_{6} as a candidate material which displays remarkable insulating behavior. Our previous results showed the presence of colossal magnetoresistance in this system, which is consistent with the presence of magnetic polarons. Here we investigate the effects of applied hydrostatic and uniaxial pressure on Eu_{5}In_{2}Sb_{6} single crystals in an effort to drive the system through a topological phase transition. Our results show an exceptionally large decrease in resistivity under applied hydrostatic pressure as well as uniaxial pressure along the c axis. 
Monday, March 2, 2020 4:18PM  4:30PM 
D59.00010: Spintocharge conversion on the edge of quantum spin Hall insulator Yasufumi Araki, Takahiro Misawa, Kentaro Nomura We present our theoretical work on dynamical spintocharge conversion at the edge of a quantum spin Hall insulator (QSHI), namely a twodimensional topological insulator with helical edge states. Interconversion between spin and chargerelated quantities has been a key idea in making use of magnetic materials, especially in the context of spintronics. QSHI is a typical system showing a universal chargetospin conversion behavior, namely the quantum spin Hall effect, whereas the spintocharge conversion therein is still not clearly understood. 
Monday, March 2, 2020 4:30PM  4:42PM 
D59.00011: Exchange interactions between topological and metamagnetic insulators Ying Wang, Valeria Lauter, Jacek K. Furdyna, Xinyu Liu, Leonid Rokhinson The hallmark of 3D topological insulators(TIs)are topological surface states(TSS)protected by the timereversal symmetry.One of the important goals is the ability to manipulate surface electron states and to lift the topological protection of TSS for the emergence of novel physical phenomena.Gapping of TSS has been demonstrated in TIs with bulk doping using the magnetic impurities and via exchange coupling to insulating magnetic or antiferromagnetic materials.Here we report an experimental study of exchange coupling in EuSe/Bi_{2}Se_{3} and EuSe/Bi_{2}Se_{3}/EuSe heterostructures grown by MBE.Bi_{2}Se_{3} is a typical TI and EuSe is a metamagnetic insulator with a rich phase diagram of paramagnetic,antiferromagnetic,ferrimagnetic and ferromagnetic phases at low temperatures.Polarized neutron scattering experiments indicate enhanced exchange interaction between EuSe and Bi_{2}Se_{3} and we observe finite inplane magnetization at the interface even above the Néel temperature (T_{N}~5K) of EuSe.In transport measurement,we observe modulation of longitudinal and Hall resistance as a function of the magnetic field,which may be explained by the formation and rearrangement of magnetic domains at the interface. 
Monday, March 2, 2020 4:42PM  4:54PM 
D59.00012: Characterization of the topological magnetoelectric effect in Bi_{2}Se_{3 }topological insulator Nezhat Pournaghavi, Anna Pertsova, Allan Macdonald, Carlo M Canali

Monday, March 2, 2020 4:54PM  5:06PM 
D59.00013: Electrically Tunable Anomalous Hall Effect in Topological Crystalline Insulator Films Peng Deng, Peng Zhang, Xiaoyu Che, KangLung Wang Introducing ferromagnetism into topological crystalline insulator SnTe could lead to the highChernnumber quantum anomalous Hall effect. Here we report the observation of magnetic proximity effect in heterostructures formed by the SnTe (111) and 2 quintuple layers thick Crdoped (BiSb)_{2}Te_{3} (CBST). Owing to the charge transfer between CBST and SnTe, the Fermi level of the latter can be tuned by adjusting the Bi/Sb ratio in the former. An anomalous Hall resistance as large as 0.08 h/e^{2} is revealed. The transferred charges modify the electric field perpendicular to the film and change the critical thickness of topological phase transition in SnTe. Such property makes SnTe useful in realizing the topological transistor. 
Monday, March 2, 2020 5:06PM  5:18PM 
D59.00014: LargeGap Quantum Anomalous Hall effect in a Magnetically doped TypeI Topological Heterostructure Anh Pham, Panchapakesan Ganesh Current method of doping a topological insulator (TI) with magnetic element has only yielded low temperature quantum anomalous hall effect due to the small exchange energy of dopants and uncontrollable shifts in the Fermilevel due to bandbending effects [1]. To overcome the bandbending effect, Fermi level can be pinned by creating a TypeI type TI/insulator band offset, while the magnetic exchange energy can be increased by searching over different potential magnetic dopants that remain neutral. Given the recent successes of growing highquality TIfilms on Cr_{2}O_{3} heterostructure [2], we study different magnetically doped TI/Cr_{2}O_{3} heterostructures using a combination of density functional theory and k.p modelling. We find Sb_{2}Te_{3} to form a typeI interface with Cr_{2}O_{3}l, thereby making the heterostructure insulating, meanwhile maximizing the Zeeman energy due to the segeration of the dopants at the interface, suggesting possible roomtemperature QAHE. We also compare our interfacial magnetism model with available experiments. 
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